1. Field of the Invention
The present invention relates to controlling gas flow from an oil producing formation. More particularly, the present invention is a method of preventing gas escape from an oil formation by sequential injection of two reactants into the formation, the product of the reaction forming a sealing zone to prevent gas breakthrough.
2. Description of the Prior Art
Hydrocarbon producing formations typically include both a gas and a companion oil bearing zone or formation. The gas, due to its low density, tends to accumulate above the liquid hydrocarbons (oil) at a horizontal interface and form a cap. This cap exerts pressure upon the oil and facilitates the extraction of the oil once a well bore is drilled and completed to the producing interval. While it is advantageous to keep the gas within the formation during oil recovery, this is not always possible due to vertical permeability within the formation or other causes.
In a typical well completion, a production pipe or tubing extends within the well bore or casing downwardly adjacent the producing formation from the surface location. The producing interval is isolated, usually through the use of well bore packers. Perforating the well casing adjacent the oil region then permits oil to flow into the production pipe from the formation.
The motive force necessary to move oil from the subsurface formation to the well surface may be supplied in a number of ways, for example, by natural reservoir pressures or by surface or subsurface pumps. At certain production rates, however, a substantial pressure gradient may be set up radially outwardly from the well bore due to flow restriction through the minute interstices of the formation. In effect, oil is withdrawn from the space immediately about the well bore at a rate greater than that at which it is readily replenished from the surrounding formation. Therefore, the level of oil progressively decreases to below the horizontal interface of the oil and gas. Thus, oil previously present in the pores of the formation just at the interface tends to be replaced by gas which flows vertically down the formation and then into the well bore, resulting in what is known as xe2x80x9cconingxe2x80x9d.
There are several methods of preventing coning and altering the permeability of formations. Most methods involve the formation of polymers or foams within the formation to reduce gas permeability. For example, Allen (U.S. Pat. No. 2,713,906) discloses a method of selectively blocking the formation approximately at the plane of the interface between the oil and gas cap. This blocking or plugging is achieved by localized, selective deposition of an asphaltic or bituminous material at the liquid-gas interface. This is accomplished by injecting asphaltic material into the formation just below the liquid surface as an asphalt-rich nonaqueous solution. The asphalt is then precipitated by contact with the gas within the formation to create a gas barrier.
Another method is disclosed in Raza (U.S. Pat. No. 3,616,858), in which MQ-type silicone is pumped into the formation. The silicon is pumped into a disk-shaped volume at the oil-gas interface by using packers, or injected into the entire formation. Contact between the gas and the silicon causes the silicon to foam and inhibits any further flow of gas. In another method disclosed in Boneau (U.S. Pat. No. 3,779,315), a polymeric solution is injected into the formation at a location lower in elevation than the gas cap. Gas is preferably removed from the gas cap during injection to facilitate movement of the polymeric material up into the zones of high permeability between the gas cap and the lower oil bearing portion of the formation. Preferred polymers include hydrolyzed polyacrylamides, polysaccharide carboxylmethyl cellulose and polyethylene oxide.
A method of altering the permeability of formation when other gases such as CO2 are injected into the formation is disclosed by Bruning et al. (U.S. Pat. Nos. 4,569,393, and 4,657,944), in which slugs of sequentially added substances are injected into the well bore. The substances are, at least, a thickened slug of sequestered polyvalent metal cation such as aluminum and a gelable polymeric viscosifier such as polyacrylamide, and CO2, to decrease the pH of the polymer slug which then induces gelation of the slug. The goal is to preferentially decrease water permeability in highly permeable zones in the formation, thus improving the efficiency of water flooding. Sandiford et al. (U.S. Pat. No. 4,673,038) also discloses a similar method using polyvinyl alcohols and other monomers with copolymers such as aldehydes to then form a polymer within the highly permeable regions of the formation. Finally, Laramay et al. (U.S. Pat. No. 5,320,173) discloses a method of preventing gas breakthrough comprising the injection of gaseous polymerizable materials such as ethylene oxide into the formation, followed by shutting the material in for a period of time to allow polymerization of the material to form a gas impermeable layer at or near the gas and oil interface.
One of the problems with preventing gas breakthrough or altering formation permeability is the fact that polymeric reactions can be complicated and difficult to control. They can be temperature sensitive, as well as pressure sensitive. Further, the materials used in these methods can be uneconomical to use. Another drawback is the insolubility of the polymers and foams with respect to the oil which can hinder oil production from parts of the formation.
What is needed is a method of preventing gas breakthrough that does not irreversibly hinder oil production, and is fast and economical to implement.
It is therefore one object of the present invention to provide an improved method of preventing gas breakthrough in hydrocarbon formations.
It is another object of the present invention to provide a means of creating a sealing zone within a gas containing oil formation that allows oil to flow through but does not allow the gas to flow.
It is yet another object of the present invention to provide a method of sealing gas within an oil well that advantageously avoids the use of polymers or foams.
These and other objects of the present invention are achieved by a method of preventing gas breakthrough in a hydrocarbon bearing formation comprising sequential injections of reactants. The method can include injecting a first reactant of a salt solution through a perforated well bore having a gas zone above an oil bearing zone. The salt solution can be alkaline earth salt solution. A spacer solution is then injected into the well bore behind the alkaline earth salt solution. Next, a second reactant of an aqueous dispersion of an oil soluble fatty acid component is injected through the well bore.
The alkaline earth salt solution and fatty acid dispersion component will react and create a precipitate or gum. The precipitate is allowed to fall by gravity through a selected region of the gas bearing zone into a selected region of the oil bearing zone to thereby form a sealed zone within the hydrocarbon bearing formation. Finally, oil is produced from the oil bearing zone through the well bore, as the fatty acid precipitate is allowed to gradually dissolve in the hydrocarbon present in the oil bearing zone as oil is being produced.
Alternately, a packer can be used to isolate the oil zone. Then, the casing above the oil zone can be perforated adjacent the gas zone. A divalent salt solution, followed by a spacer solution and then the fatty acid solution can then be injected into the gas zone. Because of their different densities, the reactants should drop into the oil zone, assuming some vertical permeability in the zones. The two reactants react to form a gas insoluble residue damaging the cone. That is, the separate reactants fall through the cone while reacting; once formed, the reaction product is a gummy, pliable reaction product. If, for some reason, the precipitate invades the oil zone, it is oil soluble and easily dissolved. Once the injection of the two reactant solutions is complete, the perforations adjacent the gas zone can be closed off, as by a squeeze cementing operation.
Typical alkaline earth salts used for the first reactant are CaCl2 and MgCl2, their concentrations in the first reactant being between 4 and 6 percent by weight in water in a preferred embodiment of the invention.
The xe2x80x9cspacer solutionxe2x80x9d used between the first and second reactant solutions is typically a 2 percent by weight solution of KCl in water.
By the term xe2x80x9cfatty acid componentxe2x80x9d, it is meant a reactant component of the invention comprising at least a fatty acid. The fatty acid component can also comprise a mixture of fatty acids, and can include other components such as fats (fatty acids condensed with glycerol), NH4OH, isopropyl alcohol, tall oils or xe2x80x9crosinsxe2x80x9d (mixtures of such components as multi-ring organic acids with fats, fatty acids, and other condensation products) or other components.
A non-limiting list of fatty acids useful as the fatty acid component of the invention includes lauric acid {CH3(CH2)10COOH}, palmitic acid {CH3(CH2)14COOH}, stearic acid {CH3(CH2)16COOH}, oleic acid {CH3(CH2)7CHxe2x95x90CH(CH2)7COOH}, linoleic acid {CH3(CH2)4CHxe2x95x90CHCH2CHxe2x95x90CH(CH2)7COOH}, linolenic acid {CH3CH2CHxe2x95x90CHCH2CHxe2x95x90CHCH2CHxe2x95x90CH(CH2)7COOH}, eleostearic acid {CH3(CH2)3CHxe2x95x90CHCHxe2x95x90CHCHxe2x95x90CH(CH2)7COOH}, licanic acid {CH3(CH2)3CHxe2x95x90CHCHxe2x95x90CHCHxe2x95x90CH(CH2)4CO (CH2)2COOH}, ricinoleic acid {CH3(CH2)5CHCOHCH2CHxe2x95x90CH(CH2)7COOH}, palmitoleic acid {CH3(CH2)5CHxe2x95x90CH(CH2)7COOH}, petroselenic acid {CH3(CH2)10CHxe2x95x90CH(CH2)4COOH}, vaccenic acid {CH3(CH2)5CHxe2x95x90CH(CH2)9COOH}, and erucic acid {CH3(CH2)7CHxe2x95x90CH(CH2)11COOH}.
Additional objects, features and advantages will be apparent in the written description which follows.